1. Field of the Invention
This invention relates broadly to surgical devices. More particularly, this invention relates to orthopedic implants, and specifically to bone plates.
2. State of the Art
Internal fixation of diaphyseal fractures using plates has been popular for a number of years. Under internal fixation, the normal callus that forms as a diaphyseal bone fracture heals is naturally fusiform (tapered at both ends). This healing process takes place, generally, according to Wolff's Law, which states that bone responds dynamically to stress and strain by altering its internal architecture.
For many years compression plates have been popular in the management of such fractures. Compression plates are held against the fractured bone with screws having shafts with cortical threads which engage the bone and heads which provide a compressive force against the plate. The plate and bone are thus forced against each other in a manner that transfers load primarily between a bone contacting surface of the plate and the bone surface to reinforce the fractured bone during healing. This manner of plating generally creates relatively low stress concentration in the bone, as there is a large contact area between the plate and the diaphyseal bone surface permitting transfer of load to be dispersed. In addition, the screws of conventional compression implants are subject only to tension loads. However, if bone quality is poor, the bone screws may not hold tightly in the bone and the internal fixation may be greatly compromised.
U.S. Pat. No. 6,001,099 to Huebner describes one type of compression plate for use with variable angle screws. The plate includes varying degrees of rigidity along its length by varying the width of the plate at selected distances from the center of the plate. This purportedly operates to limit refracture of bones at the ends of the plates during the healing process. Other compression plates are structured to have decreased rigidity at their ends by having a tapered fusiform shape.
More recently fixed angle plates have been used to stabilize fractures. In distinction from compression plates, fixed angle plates generally do not reinforce a fractured bone by compressing the plate against the bone. Rather, such plates use fixed cortical screws fixed relative to the plate. Commonly the screw holes and screw heads are threaded together to lock the screws within the screw holes. As the fixed angle screws are directly coupled to the plate and extend into the bone in a fixed angle arrangement, the screws provide a stabilizing framework even in osteopenic bone, while the plate functions as an internal splint to facilitate proper healing. It is generally not desirable to taper the ends of a fixed angle plate. Given that there is no compression, tapered ends of the plate could allow the plate to rock on the bone.
Surgeons have begun to notice a significant clinical problem with the occurrence of particular refractures with fixed angle diaphyseal plates that are more common than with conventional compression plates. The fractures occur with minimal trauma at the junction between the plates and the intact bone and are not at the original fracture site. They are generally located at the bone holes near the ends of a plate, which may be subject to more of the torsional load than the bone holes in the mid portion of the plate.
One explanation for the problem is that the stabilizing framework provided by fixed angle implant is too rigid and creates unacceptably high stress concentrations at the holes in the bone at the ends of the plate. Typically these fractures occur in a spiral or bivalve type of fracture pattern, which suggests that torsional loads are frequently to blame. By themselves, fixed angle diaphyseal plates are just as rigid as conventional plates, but fixed angle screws are generally larger in diameter and stiffer than compressive-type screws. While the fixed angle screws are not subject to loads under tension, they are required to transmit torsional, bending and shear forces which conventional compression screws do not. Moreover, while fixed angle screws are generally larger than variable angle screws for the same application, a fixed angle plate transfers the loads from the bone to the plate by means of the screws which present a remarkably smaller area of contact with bone, i.e., the inside surface of the holes in the bone in which the screws reside, in distinction from the lower bone contacting surface of a conventional compression plate. This produces very high stress concentrations within the bone at the locations of the screws.